Vaccinia virus inhibitors as a paradigm for the chemotherapy of poxvirus infections

A scientific career from the early 1960s till 2023: A tale of the various protagonists

In this era spanning more than 60 years (from the early 1960s till today (2023), a broad variety of actors played a decisive role: Piet De Somer, Tom C. Merigan, Paul A. Janssen, Maurice Hilleman, and Georges Smets. Two protagonists (Antonín Holý and John C. Martin) formed with me a unique triangle (the Holý Trinity). Walter Fiers' group (with the help of Jean Content) contributed to the cloning of human β-interferon, and Piet Herdewijn accomplished the chemical synthesis of an array of anti-HIV 2',3'-dideoxynucleoside analogues. Rudi Pauwels, Masanori Baba, Dominique Schols, Johan Neyts, Lieve Naesens, Anita Van Lierde, Graciela Andrei, Robert Snoeck and Dirk Daelemans, as members of my team, helped me in achieving the intended goal, the development of a selective therapy for virus infections. The collaboration with "Lowie" (Guangdi Li) generated a new dimension for the future.

Selected Milestones in Antiviral Drug Development

This review article will describe the (wide) variety of approaches that I envisaged to develop a specific therapy for viral infections: (i) interferon and its inducers, (ii) HSV, VZV and CMV inhibitors, (iii) NRTIs (nucleoside reverse transcriptase inhibitors), NtRTIs (nucleotide reverse transcriptase inhibitors) and NNRTIs (non-nucleoside reverse transcriptase inhibitors) as HIV inhibitors, (iv) NtRTIs as HBV inhibitors, and finally, (v) the transition of an HIV inhibitor to a stem cell mobilizer, as exemplified by AMD-3100 (Mozobil®).

The Overview of Potential Antiviral Bioactive Compounds in Poxviruses

Poxviruses belong to the family of double-stranded DNA viruses, and it is pathogenic for humans and spread worldwide. These viruses cause infections and various diseases in human. So, it is required to develop new drugs for the treatment of smallpox or other poxvirus infections. Very few potential compounds for the treatment of poxvirus such as smallpox, chickenpox, and monkeypox have been reported. Most of the compounds has used as vaccines. Cidofovir is most commonly used as a vaccine for the treatment of poxviruses. There are no phytochemicals reported for the treatment of poxviruses. Very few phytochemicals are under investigation for the treatment of poxviruses.

An Overview of Antivirals against Monkeypox Virus and Other Orthopoxviruses

The current monkeypox outbreaks during the COVID-19 pandemic have reignited interest in orthopoxvirus antivirals. Monkeypox belongs to the Orthopoxvirus genus of the Poxviridae family, which also includes the variola virus, vaccinia virus, and cowpox virus. Two orally bioavailable drugs, tecovirimat and brincidofovir, have been approved for treating smallpox infections. Given their human safety profiles and in vivo antiviral efficacy in animal models, both drugs have also been recommended to treat monkeypox infection. To facilitate the development of additional orthopoxvirus antivirals, we summarize the antiviral activity, mechanism of action, and mechanism of resistance of orthopoxvirus antivirals. This perspective covers both direct-acting and host-targeting antivirals with an emphasis on drug candidates showing in vivo antiviral efficacy in animal models. We hope to speed the orthopoxvirus antiviral drug discovery by providing medicinal chemists with insights into prioritizing proper drug targets and hits for further development.

Potential therapeutic targets for Mpox: the evidence to date

INTRODUCTION

The global Mpox (MPX) disease outbreak caused by the Mpox virus (MPXV) in 2022 alarmed the World Health Organization (WHO) and health regulation agencies of individual countries leading to the declaration of MPX as a Public Health Emergency. Owing to the genetic similarities between smallpox-causing poxvirus and MPXV, vaccine JYNNEOS, and anti-smallpox drugs Brincidofovir and Tecovirimat were granted emergency use authorization by the United States Food and Drug Administration. The WHO also included cidofovir, NIOCH-14, and other vaccines as treatment options.

AREAS COVERED

This article covers the historical development of EUA-granted antivirals, resistance to these antivirals, and the projected impact of signature mutations on the potency of antivirals against currently circulating MPXV. Since a high prevalence of MPXV infections in individuals coinfected with HIV and MPXV, the treatment results among these individuals have been included.

EXPERT OPINION

All EUA-granted drugs have been approved for smallpox treatment. These antivirals show good potency against Mpox. However, conserved resistance mutation positions in MPXV and related poxviruses, and the signature mutations in the 2022 MPXV can potentially compromise the efficacy of the EUA-granted treatments. Therefore, MPXV-specific medications are required not only for the current but also for possible future outbreaks.

Therapeutic strategies for human poxvirus infections: Monkeypox (mpox), smallpox, molluscipox, and orf

Therapeutic and vaccine development for human poxvirus infections (e.g., monkeypox (mpox) virus, variola virus, molluscum contagiosum virus, orf virus) has been largely deserted, especially after the eradication of smallpox by 1980. Human mpox is a self-limited disease confined to Central and West Africa for decades. However, since April 2022, mpox has quickly emerged as a multi-country outbreak, urgently calling for effective antiviral agents and vaccines to control mpox. Here, this review highlights possible therapeutic options (e.g., tecovirimat, brincidofovir, cidofovir) and other strategies (e.g., vaccines, intravenous vaccinia immune globulin) for the management of human poxvirus infections worldwide.

Comprehensive literature review of monkeypox

The current outbreak of monkeypox (MPX) infection has emerged as a global matter of concern in the last few months. MPX is a zoonosis caused by the MPX virus (MPXV), which is one of the Orthopoxvirus species. Thus, it is similar to smallpox caused by the variola virus, and smallpox vaccines and drugs have been shown to be protective against MPX. Although MPX is not a new disease and is rarely fatal, the current multi-country MPX outbreak is unusual because it is occurring in countries that are not endemic for MPXV. In this work, we reviewed the extensive literature available on MPXV to summarize the available data on the major biological, clinical and epidemiological aspects of the virus and the important scientific findings. This review may be helpful in raising awareness of MPXV transmission, symptoms and signs, prevention and protective measures. It may also be of interest as a basis for performance of studies to further understand MPXV, with the goal of combating the current outbreak and boosting healthcare services and hygiene practices.Trial registration: ClinicalTrials.gov identifier: NCT02977715..Trial registration: ClinicalTrials.gov identifier: NCT03745131..Trial registration: ClinicalTrials.gov identifier: NCT00728689..Trial registration: ClinicalTrials.gov identifier: NCT02080767..

Cidofovir for the Treatment of Molluscum Contagiosum Virus

That cidofovir, an acyclic nucleoside phosphonate (ANP), was inhibitory to the replication of poxviruses was first demonstrated by De Clercq et al.. That its active metabolite, the diphosphate, was found to be inhibitory to the molluscum contagiosum (M. contagiosum) DNA polymerase was demonstrated by Watanabe and Tamaki. Twelve different independent observations have then indicated that cidofovir administered intravenously, topically or intralesionally is efficacious in the treatment of M. contagiosum mostly in immunosuppressed patients.

Antiviral activity of Apigenin against buffalopox: Novel mechanistic insights and drug-resistance considerations

We describe herein that Apigenin, which is a dietary flavonoid, exerts a strong in vitro and in ovo antiviral efficacy against buffalopox virus (BPXV). Apigenin treatment was shown to inhibit synthesis of viral DNA, mRNA and proteins, without affecting other steps of viral life cycle such as attachment, entry and budding. Although the major mode of antiviral action of Apigenin was shown to be mediated via targeting certain cellular factors, a modest inhibitory effect of Apigenin was also observed directly on viral polymerase. We also evaluated the selection of drug-resistant virus variants under long-term selection pressure of Apigenin. Wherein Apigenin-resistant mutants were not observed up to ~ P20 (passage 20), a significant resistance was observed to the antiviral action of Apigenin at ~ P30. However, a high degree resistance could not be observed even up to P60. To the best of our knowledge, this is the first report describing in vitro and in ovo antiviral efficacy of Apigenin against poxvirus infection. The study also provides mechanistic insights on the antiviral activity of Apigenin and selection of potential Apigenin-resistant mutants upon long-term culture.

The proline synthesis enzyme P5CS forms cytoophidia in Drosophila

Compartmentation of enzymes via filamentation has arisen as a mechanism for the regulation of metabolism. In 2010, three groups independently reported that CTP synthase (CTPS) can assemble into a filamentous structure termed the cytoophidium. In searching for CTPS-interacting proteins, here we perform a yeast two-hybrid screening of Drosophila proteins and identify a putative CTPS-interacting protein, △¹-pyrroline-5-carboxylate synthase (P5CS). Using the Drosophila follicle cell as the in vivo model, we confirm that P5CS forms cytoophidia, which are associated with CTPS cytoophidia. Overexpression of P5CS increases the length of CTPS cytoophidia. Conversely, filamentation of CTPS affects the morphology of P5CS cytoophidia. Finally, in vitro analyses confirm the filament-forming property of P5CS. Our work links CTPS with P5CS, two enzymes involved in the rate-limiting steps in pyrimidine and proline biosynthesis, respectively.

Human dihydrofolate reductase and thymidylate synthase form a complex in vitro and co-localize in normal and cancer cells

Enzymes involved in thymidylate biosynthesis, thymidylate synthase (TS), and dihydrofolate reductase (DHFR) are well-known targets in cancer chemotherapy. In this study, we demonstrated for the first time, that human TS and DHFR form a strong complex in vitro and co-localize in human normal and colon cancer cell cytoplasm and nucleus. Treatment of cancer cells with methotrexate or 5-fluorouracil did not affect the distribution of either enzyme within the cells. However, 5-FU, but not MTX, lowered the presence of DHFR-TS complex in the nucleus by 2.5-fold. The results may suggest the sequestering of TS by FdUMP in the cytoplasm and thereby affecting the translocation of DHFR-TS complex to the nucleus. Providing a strong likelihood of DHFR-TS complex formation in vivo, the latter complex is a potential new drug target in cancer therapy. In this paper, known 3D structures of human TS and human DHFR, and some protozoan bifunctional DHFR-TS structures as templates, are used to build an in silico model of human DHFR-TS complex structure, consisting of one TS dimer and two DHFR monomers. This complex structure may serve as an initial 3D drug target model for prospective inhibitors targeting interfaces between the DHFR and TS enzymes.

Improved safety of a replication-competent poxvirus-based HIV vaccine with the introduction of the HSV-TK/GCV suicide gene system

INTRODUCTION

Replication-competent vaccinia viruses (VACVs) show prolonged antigen expression time and greater stimulation of immune responses than their replication-incompetent counterparts. However, there is the potential risk of serious post-vaccination complications, especially for children and immunocompromised individuals, leading to safety concerns about the reintroduction of VACV as a vaccine vector. In this study, we improved the safety of the vaccinia virus TianTan (VACV-TT) based HIV vaccine by introducing the HSV-TK/GCV suicide gene system, which is composed of the herpes simplex virus type 1 thymidine kinase gene (HSV-tk) and the antiviral drug ganciclovir (GCV).

MATERIALS AND METHODS

By inserting the HSV-tk gene into the replication-competent VACV-TT genome, a new vector, TT-TK (VACV-TT expressing the HSV-tk gene), and a candidate vaccine, TT-EnvTK (TT-TK expressing the HIV-1 env gene), were constructed.

RESULTS

The new vector TT-TK exhibited reduced replication capacity both in vitro and in vivo in the presence of GCV. GCV inhibited the replication of TT-TK in the brains of mice and skin of rabbits, and provided 100% protection in mice against lethal challenge with TT-TK at a dose of 80mg/kg/day. Furthermore, the candidate vaccine TT-EnvTK induced cellular and humoral immunity against HIV-1 antigen that was comparable to the immunity induced by VTKgpe (VACV-TT expressing HIV-1 env, gag, and pol genes).

DISCUSSION

These promising results suggest a new strategy to mitigate the potential risk of post-vaccination complications from replication-competent VACV-based HIV vaccines.

C-Nucleosides To Be Revisited

Two new C-nucleoside analogues, BCX4430, an imino-C-nucleoside, and GS-6620, a phosphoramidate derivative of 1'-cyano-2'-C-methyl-4-aza-7,9-dideazaadenosine C-nucleoside, have been recently described as effective against filovirus infections (Marburg) and hepatitis C virus (HCV), respectively. The first C-nucleoside analogues were described about half a century ago. The C-nucleoside pseudouridine is a natural component of RNA, and various other C-nucleoside analogues have been reported previously for their antiviral and/or anticancer potential, the most prominent being pyrazofurin, tiazofurin, and selenazofurin. In the meantime, showdomycin, formycin, and various triazole, pyrazine, pyridine, dihydroxyphenyl, thienopyrimidine, pyrazolotriazine, and porphyrin C-nucleoside analogues have been described. It would be worth revisiting these C-nucleosides and derivatives thereof, including their phosphoramidates, for their therapeutic potential in the treatment of virus infections and, where appropriate, cancer as well.

Design of inhibitors of ODCase

ODCase is a highly proficient enzyme responsible for the decarboxylation of orotidine monophosphate to generate uridine monophosphate. ODCase has attracted early attention due to its interesting mechanism of catalysis. In order to exploit therapeutic advantages due to the inhibition of ODCase, one must have selective inhibitors of this enzyme from the pathogen, or a dysregulated molecular mechanism involving ODCase. ODCase inhibitors have potential applications as anticancer agents, antiviral agents, antimalarial agents and potentially act against other parasitic diseases. A variety of C6-substituted uridine monophosphate derivatives have shown excellent inhibition of ODCase. 6-iodouridine is a potent inhibitor of the malaria parasite, and its monophosphate form covalently inhibits ODCase. A variety of inhibitors of ODCase with potential applications as therapeutic agents are discussed in this review.

A cutting-edge view on the current state of antiviral drug development

Prominent in the current stage of antiviral drug development are: (i) for human immunodeficiency virus (HIV), the use of fixed-dose combinations (FDCs), the most recent example being Stribild(TM); (ii) for hepatitis C virus (HCV), the pleiade of direct-acting antivirals (DAAs) that should be formulated in the most appropriate combinations so as to obtain a cure of the infection; (iii)-(v) new strategies (i.e., AIC316, AIC246, and FV-100) for the treatment of herpesvirus infections: herpes simplex virus (HSV), cytomegalovirus (CMV), and varicella-zoster virus (VZV), respectively; (vi) the role of a new tenofovir prodrug, tenofovir alafenamide (TAF) (GS-7340) for the treatment of HIV infections; (vii) the potential use of poxvirus inhibitors (CMX001 and ST-246); (viii) the usefulness of new influenza virus inhibitors (peramivir and laninamivir octanoate); (ix) the position of the hepatitis B virus (HBV) inhibitors [lamivudine, adefovir dipivoxil, entecavir, telbivudine, and tenofovir disoproxil fumarate (TDF)]; and (x) the potential of new compounds such as FGI-103, FGI-104, FGI-106, dUY11, and LJ-001 for the treatment of filoviruses (i.e., Ebola). Whereas for HIV and HCV therapy is aimed at multiple-drug combinations, for all other viruses, HSV, CMV, VZV, pox, influenza, HBV, and filoviruses, current strategies are based on the use of single compounds.

Only one isoform of Drosophila melanogaster CTP synthase forms the cytoophidium

CTP synthase is an essential enzyme that plays a key role in energy metabolism. Several independent studies have demonstrated that CTP synthase can form an evolutionarily conserved subcellular structure termed cytoophidium. In budding yeast, there are two isoforms of CTP synthase and both isoforms localize in cytoophidium. However, little is known about the distribution of CTP synthase isoforms in Drosophila melanogaster. Here, we report that three transcripts generated at the CTP synthase gene locus exhibit different expression profiles, and three isoforms encoded by this gene locus show a distinct subcellular distribution. While isoform A localizes in the nucleus, isoform B distributes diffusely in the cytoplasm, and only isoform C forms the cytoophidium. In the two isoform C-specific mutants, cytoophidia disappear in the germline cells. Although isoform A does not localize to the cytoophidium, a mutation disrupting mostly isoform A expression results in the disassembly of cytoophidia. Overexpression of isoform C can induce the growth of the cytoophidium in a cell-autonomous manner. Ectopic expression of the cytoophidium-forming isoform does not cause any defect in the embryos. In addition, we identify that a small segment at the amino terminus of isoform C is necessary but not sufficient for cytoophidium formation. Finally, we demonstrate that an excess of the synthetase domain of CTP synthase disrupts cytoophidium formation. Thus, the study of multiple isoforms of CTP synthase in Drosophila provides a good opportunity to dissect the biogenesis and function of the cytoophidum in a genetically tractable organism.

DNA and RNA are made up from basic building blocks called nucleotides. Those nucleotides also play essential roles in many other biological processes. To separate biological processes within a cell is an important feature of all cell types. For example, mitochondria are specialized structures that contain ATP synthase, the enzyme that makes the nucleotide ATP. While mitochondria and ATP synthase have been studied for about 100 years, it was only very recently that we realized that there are specialized subcellular structures that contain CTP synthase, the enzyme that makes up another basic nucleotide CTP. Several independent studies have shown that CTP synthase molecules can form a filamentous structure called the cytoophidium (meaning “cellular snake” in Greek) or CTP synthase filament in bacteria, budding yeasts, fruit flies, and rat and human cells. In budding yeast, there are two isoforms of CTP synthase and both isoforms localize in the cytoophidium. Here, we report that three CTP synthase isoforms in fruit flies show a distinct subcellular distribution and only one isoform forms the cytoophidium. Thus, the study of multiple isoforms of CTP synthase in the fruit fly gives us a good way to begin to learn how and why CTP synthase molecules form this snake-like structure.

Orthopoxvirus targets for the development of new antiviral agents

Investments in the development of new drugs for orthopoxvirus infections have fostered new avenues of research, provided an improved understanding of orthopoxvirus biology and yielded new therapies that are currently progressing through clinical trials. These broad-based efforts have also resulted in the identification of new inhibitors of orthopoxvirus replication that target many different stages of viral replication cycle. This review will discuss progress in the development of new anti-poxvirus drugs and the identification of new molecular targets that can be exploited for the development of new inhibitors. The prototype of the orthopoxvirus group is vaccinia virus and its replication cycle will be discussed in detail noting specific viral functions and their associated gene products that have the potential to serve as new targets for drug development. Progress that has been achieved in recent years should yield new drugs for the treatment of these infections and might also reveal new approaches for antiviral drug development with other viruses.

Identification of a pyridopyrimidinone inhibitor of orthopoxviruses from a diversity-oriented synthesis library

Orthopoxviruses include the prototypical vaccinia virus, the emerging infectious agent monkeypox virus, and the potential biothreat variola virus (the causative agent of smallpox). There is currently no FDA-approved drug for humans infected with orthopoxviruses. We screened a diversity-oriented synthesis library for new scaffolds with activity against vaccinia virus. This screen identified a nonnucleoside analog that blocked postreplicative intermediate and late gene expression. Viral genome replication was unaffected, and inhibition could be elicited late in infection and persisted upon drug removal. Sequencing of drug-resistant viruses revealed mutations predicted to be on the periphery of the highly conserved viral RNA polymerase large subunit. Consistent with this, the compound had broad-spectrum activity against orthopoxviruses in vitro. These findings indicate that novel chemical synthesis approaches are a potential source for new infectious disease therapeutics and identify a potentially promising candidate for development to treat orthopoxvirus-infected individuals.

Glutamine analogs promote cytoophidium assembly in human and Drosophila cells

CTP synthase is compartmentalized within a subcellular structure, termed the cytoophidium, in a range of organisms including bacteria, yeast, fruit fly and rat. Here we show that CTP synthase is also compartmentalized into cytoophidia in human cells. Surprisingly, the occurrence of cytoophidia in human cells increases upon treatment with a glutamine analog 6-diazo-5-oxo-l-norleucine (DON), an inhibitor of glutamine-dependent enzymes including CTP synthase. Experiments in flies confirmed that DON globally promotes cytoophidium assembly. Clonal analysis via CTP synthase RNA interference in somatic cells indicates that CTP synthase expression level is critical for the formation of cytoophidia. Moreover, DON facilitates cytoophidium assembly even when CTP synthase level is low. A second glutamine analog azaserine also promotes cytoophidum formation. Our data demonstrate that glutamine analogs serve as useful tools in the study of cytoophidia.

Developments in antivirals against influenza, smallpox and hemorrhagic fever viruses

INTRODUCTION

the search for effective inhibitors to multiple infectious agents including influenza, smallpox and hemorrhagic fever viruses is an area of active research as many of these agents pose dramatic health and economic challenges to the human population. Many of these infectious agents are not only endemic threats in different parts of the globe, but are also considered to have the potential of being used as bioterrorism agents.

AREAS COVERED

this review focuses on inhibitors that are currently in use in the research community against specific emerging infectious agents and those that have bioterrorism potential. The paper provides information about the availability of FDA approved drugs, whenever applicable, and insights into the specific aspect of the agent life cycle that is affected by drug treatment, when known.

EXPERT OPINION

the key message that is conveyed in this review is that a combination of pathogen and host-based inhibitors may have to be used for successful control of viral replication to limit the development of drug resistance.

Antiviral pharmacodynamics in hollow fibre bioreactors

Pharmacodynamic investigation of antiviral compounds studies the relationship between drug exposure and the virological response. These studies are usually performed in animals and, eventually, in humans and are a very expensive proposition. To find a more efficient and less expensive method for determining pharmacodynamics of antiviral and antimicrobial compounds, the hollow fibre infection model (HFIM) system was developed to perform pharmacodynamic studies in vitro. This review covers the authors' studies on the use of in vitro hollow fibre bioreactor technologies for determining the pharmacodynamics of antiviral compounds for viruses grown in cultured cells, including HIV grown in CD4+ lymphoblastoid cells, vaccinia viruses grown in HeLa-S3 cells and influenza viruses grown in Madin-Darby canine kidney cells. Where possible, correlations between the pharmacodynamic index derived from the in vitro HFIM systems and clinical pharmacodynamic studies are made.

Broad-spectrum antiviral activity including human immunodeficiency and hepatitis C viruses mediated by a novel retinoid thiosemicarbazone derivative

Aromatic aldehyde-derived thiosemicarbazones 4-6, the S-substituted modified thiosemicarbazones 7/8, and a vitamin A-derived (retinoid) thiosemicarbazone derivative 12 were investigated as inhibitors of human hepatitis C virus (HCV) subgenomic RNA replicon Huh7 ET (luc-ubi-neo/ET) replication. Compounds 4-6 and 12 were found to be potent suppressors of HCV RNA replicon replication. The trifluoromethoxy-substituted thiosemicarbazone 6 and the retinoid thiosemicarbazone derivative 12 were even superior in selectivity to the included reference agent recombinant human alpha-interferon-2b, showing potencies in the nanomolar range of concentration. In addition, compounds 5, 6, 8 and 12 were tested as inhibitors of cytopathic effect (CPE) induced by human varicella-zoster virus (VZV) and/or human cytomegalovirus (HCMV). Compounds 4-6, 8 and 12 were additionally examined as inhibitors of CPE induced by cowpox virus and vaccinia virus. Thiosemicarbazone 4 was inhibitory on cowpox and vaccinia virus replication comparable in potency and selectivity to the reference agent cidofovir. Retinoid thiosemicarbazone derivative 12 was active as micromolar inhibitor of VZV, HCMV, and, in addition, human immunodeficiency virus type 1 (HIV-1) replication. These results indicate that thiosemicarbazone derivatives are appropriate lead structures to be evaluated in targeted antiviral therapies for hepatitis C (STAT-C), and that the vitamin A-related thiosemicarbazone derivative 12 emerges as a broad-spectrum antiviral agent, co-suppressing the multiplication of important RNA and DNA viruses.

Antiviral activity of Bignoniaceae species occurring in the State of Minas Gerais (Brazil): part 1

AIMS

To evaluate the antiviral activity of Bignoniaceae species occurring in the state of Minas Gerais, Brazil.

METHODS AND RESULTS

Ethanol extracts of different anatomical parts of bignoniaceous plant species have been evaluated in vitro against human herpesvirus type 1 (HSV-1), vaccinia virus (VACV) and murine encephalomyocarditis virus (EMCV) by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. A total of 34 extracts from 18 plant species selected according to ethnopharmacological and taxonomic criteria were screened. Fifteen of the 34 extracts (44.1%) have disclosed antiviral activity against one or more of the viruses assayed with EC(50) values in the range of 23.2 ± 2.5-422.7 ± 10.9 μg ml(-1).

CONCLUSIONS

Twelve of the 34 extracts (35.3%) might be considered promising sources of antiviral natural products, as they have shown EC50 ≤ 100 μg ml(-1). The present screening discloses the high potential of the Bignoniaceae family as source of antiviral agents.

SIGNIFICANCE AND IMPACT OF THE STUDY

Active extracts were identified and deserve bioguided studies for the isolation of antiviral compounds and studies on mechanism of action.

Synthesis of ester prodrugs of 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine (HPMPDAP) as anti-poxvirus agents

9-(S)-[3-Hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine (HPMPDAP) and its cyclic form were selected for further evaluation as potential drug candidates against poxvirus infections. To increase bioavailability of these compounds, synthesis of their structurally diverse ester prodrugs was carried out: alkoxyalkyl (hexadecyloxypropyl, octadecyloxyethyl, hexadecyloxyethyl), pivaloyloxymethyl (POM), 2,2,2-trifluoroethyl, butylsalicylyl, and prodrugs based on peptidomimetics. Most HPMPDAP prodrugs were synthesized in the form of monoesters as well as the corresponding cyclic phosphonate esters. The activity was evaluated not only against vaccinia virus but also against different herpes viruses. The most potent and active prodrugs against vaccinia virus were the alkoxyalkyl ester derivatives of HPMPDAP, with 50% effective concentrations 400-600-fold lower than those of the parent compound. Prodrugs based on peptidomimetics, the 2,2,2-trifluoroethyl, the POM, and the butylsalicylyl derivatives, were able to inhibit vaccinia virus replication at 50% effective concentrations that were equivalent or ∼10-fold lower than those observed for the parent compounds.

Use of the Aerosol Rabbitpox Virus Model for Evaluation of Anti-Poxvirus Agents

Smallpox is an acute disease caused by infection with variola virus that has had historic effects on the human population due to its virulence and infectivity. Because variola remains a threat to humans, the discovery and development of novel pox therapeutics and vaccines has been an area of intense focus. As variola is a uniquely human virus lacking a robust animal model, the development of rational therapeutic or vaccine approaches for variola requires the use of model systems that reflect the clinical aspects of human infection. Many laboratory animal models of poxviral disease have been developed over the years to study host response and to evaluate new therapeutics and vaccines for the treatment or prevention of human smallpox. Rabbitpox (rabbitpox virus infection in rabbits) is a severe and often lethal infection that has been identified as an ideal disease model for the study of poxviruses in a non-rodent species. The aerosol infection model (aerosolized rabbitpox infection) embodies many of the desired aspects of the disease syndrome that involves the respiratory system and thus may serve as an appropriate model for evaluation of antivirals under development for the therapeutic treatment of human smallpox. In this review we summarize the aerosol model of rabbitpox, discuss the development efforts that have thus far used this model for antiviral testing, and comment on the prospects for its use in future evaluations requiring a poxviral model with a focus on respiratory infection.

Yet another ten stories on antiviral drug discovery (part D): paradigms, paradoxes, and paraductions

This review article presents the fourth part (part D) in the series of stories on antiviral drug discovery. The stories told in part D focus on: (i) the cyclotriazadisulfonamide compounds; (ii) the {5-[(4-bromophenylmethyl]-2-phenyl-5H-imidazo[4,5-c]pyridine} compounds; (iii) (1H,3H-thiazolo[3,4-a]benzimidazole) derivatives; (iv) T-705 (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) and (v) its structurally closely related analogue pyrazine 2-carboxamide (pyrazinamide); (vi) new strategies for the treatment of hemorrhagic fever virus infections, including, as the most imminent, (vii) dengue fever, (viii) the veterinary use of acyclic nucleoside phosphonates; (ix) the potential (off-label) use of cidofovir in the treatment of papillomatosis, particularly RRP (recurrent respiratory papillomatosis); and (x) finally, the prophylactic use of tenofovir to prevent HIV infections.

Antiviral Activity of 4'-thioIDU and Thymidine Analogs against Orthopoxviruses

The search for effective therapies for orthopoxvirus infections has identified diverse classes of molecules with antiviral activity. Pyrimidine analogs, such as 5-iodo-2'-deoxyuridine (idoxuridine, IDU) were among the first compounds identified with antiviral activity against a number of orthopoxviruses and have been reported to be active both in vitro and in animal models of infection. More recently, additional analogs have been reported to have improved antiviral activity against orthopoxviruses including several derivatives of deoxyuridine with large substituents in the 5 position, as well as analogs with modifications in the deoxyribose moiety including (north)-methanocarbathymidine, and 5-iodo-4'-thio-2'-deoxyuridine (4'-thioIDU). The latter molecule has proven to have good antiviral activity against the orthopoxviruses both in vitro and in vivo and has the potential to be an effective therapy in humans.

Historical perspectives in the development of antiviral agents against poxviruses

The poxvirus vaccinia virus (VV) served as the model virus for which the first antivirals, the thiosemicarbazones, were identified. This dates back to 1950; and, although there is at present no single antiviral drug specifically licensed for the chemotherapy or -prophylaxis of poxvirus infections, numerous candidate compounds have been described over the past 50 years. These compounds include interferon and inducers thereof (i.e., polyacrylic acid), 5-substituted 2’-deoxyuridines (i.e., idoxuridine), IMP dehydrogenase inhibitors, S-adenosylhomocysteine hydrolase inhibitors, acyclic nucleoside phosphonates (such as cidofovir) and alkoxyalkyl prodrugs thereof (such as CMX001), viral egress inhibitors (such as tecovirimat), and cellular kinase inhibitors (such as imatinib).

Structural determinants for the inhibitory ligands of orotidine-5'-monophosphate decarboxylase

In recent years, orotidine-5'-monophosphate decarboxylase (ODCase) has gained renewed attention as a drug target. As a part of continuing efforts to design novel inhibitors of ODCase, we undertook a comprehensive study of potent, structurally diverse ligands of ODCase and analyzed their structural interactions in the active site of ODCase. These ligands comprise of pyrazole or pyrimidine nucleotides including the mononucleotide derivatives of pyrazofurin, barbiturate ribonucleoside, and 5-cyanouridine, as well as, in a computational approach, 1,4-dihydropyridine-based non-nucleoside inhibitors such as nifedipine and nimodipine. All these ligands bind in the active site of ODCase exhibiting distinct interactions paving the way to design novel inhibitors against this interesting enzyme. We propose an empirical model for the ligand structure for rational modifications in new drug design and potentially new lead structures.

Another ten stories in antiviral drug discovery (part C): "Old" and "new" antivirals, strategies, and perspectives

The ten stories told here deal with (i) ribavirin as an inhibitor of IMP dehydrogenase and (ii) ribavirin, in combination with pegylated interferon, as the present "standard of care" for hepatitis C; (iii) S-adenosylhomocysteine hydrolase inhibitors as antiviral agents; (iv) new adamantadine derivatives for the treatment of influenza A virus infections; (v) 5-substituted 2'-deoxyuridines (i.e. IDU, TFT) for the treatment of herpes simplex virus (HSV) infections; (vi) acyclic guanosine analogues (e.g. acyclovir) for the treatment of HSV infections; (vii) OMP decarboxylase inhibitors (i.e. pyrazofurin) and CTP synthetase inhibitors (i.e. cyclopentenylcytosine) as possible antiviral agents; (viii) the future of cidofovir (and alkoxyalkyl esters thereof) and ST-246 as potential antipoxvirus agents; (ix) the two decade journey from tivirapine to rilpivirine in the ultimate therapy of HIV infections; and (x) the extension of the therapeutic application of tenofovir disoproxil fumarate (Viread) to the treatment of hepatitis B virus infection, in addition to HIV infection.

Activities of certain 5-substituted 4'-thiopyrimidine nucleosides against orthopoxvirus infections

As part of a program to identify new compounds that have activity against orthopoxviruses, a number of 4'-thionucleosides were synthesized and evaluated for their efficacies against vaccinia and cowpox viruses. Seven compounds that were active at about 1 microM against both viruses in human cells but that did not have significant toxicity were identified. The 5-iodo analog, 1-(2-deoxy-4-thio-beta-d-ribofuranosyl)-5-iodouracil (4'-thioIDU), was selected as a representative molecule; and this compound also inhibited viral DNA synthesis at less than 1 microM but only partially inhibited the replication of a recombinant vaccinia virus that lacked a thymidine kinase. This compound retained complete activity against cidofovir- and ST-246-resistant mutants. To determine if this analog had activity in an animal model, mice were infected intranasally with vaccinia or cowpox virus and treatment with 4'-thioIDU was given intraperitoneally or orally twice daily at 50, 15, 5, or 1.5 mg/kg of body weight beginning at 24 to 120 h postinfection and was continued for 5 days. Almost complete protection (87%) was observed when treatment with 1.5 mg/kg was begun at 72 h postinfection, and significant protection (73%) was still obtained when treatment with 5 mg/kg was initiated at 96 h. Virus titers in the liver, spleen, and kidney were reduced by about 4 log(10) units and about 2 log(10) units in mice infected with vaccinia virus and cowpox virus, respectively. These results indicate that 4'-thioIDU is a potent, nontoxic inhibitor of orthopoxvirus replication in cell culture and experimental animal infections and suggest that it may have potential for use in the treatment of orthopoxvirus infections in animals and humans.

SOCS-1 mimetics protect mice against lethal poxvirus infection: identification of a novel endogenous antiviral system

The suppressor of cytokine signaling 1 (SOCS-1) protein modulates cytokine signaling by binding to and inhibiting the function of Janus kinases (JAKs), ErbB, and other tyrosine kinases. We have developed a small tyrosine kinase inhibitor peptide (Tkip) that binds to the autophosphorylation site of tyrosine kinases and inhibits activation of STAT transcription factors. We have also shown that a peptide corresponding to the kinase-inhibitory region of SOCS-1, SOCS1-KIR, similarly interacts with the activation loop of JAK2 and blocks STAT activation. Poxviruses activate cellular tyrosine kinases, such as ErbB-1 and JAK2, in the infection of cells. We used the pathogenesis of vaccinia virus in C57BL/6 mice to determine the ability of the SOCS-1 mimetics to protect mice against lethal vaccinia virus infection. Injection of mice intraperitoneally with Tkip or SOCS1-KIR containing a palmitate for cell penetration, before and at the time of intranasal challenge with 2 x 10(6) PFU of vaccinia virus, resulted in complete protection at 100 microg. Initiation of treatment 1 day postinfection resulted in 80% survival. Administration of SOCS-1 mimetics by the oral route also protected mice against lethal effects of the virus. Both SOCS1-KIR and Tkip inhibited vaccinia virus transcription and replication at early and possibly later stages of infection. Vaccinia virus-induced phosphorylation of ErbB-1 and JAK2 was inhibited by the mimetics. Protected mice mounted a strong humoral and cellular response to vaccinia virus. The use of SOCS-1 mimetics in the treatment of poxvirus infections reveals an endogenous regulatory system that previously was not known to have an antiviral function.

Crystal structure of poxvirus thymidylate kinase: an unexpected dimerization has implications for antiviral therapy

Unlike most DNA viruses, poxviruses replicate in the cytoplasm of host cells. They encode enzymes needed for genome replication and transcription, including their own thymidine and thymidylate kinases. Some herpes viruses encode only 1 enzyme catalyzing both reactions, a peculiarity used for prodrug activation to obtain maximum specificity. We have solved the crystal structures of vaccinia virus thymidylate kinase bound to TDP or brivudin monophosphate. Although the viral and human enzymes have similar sequences (42% identity), they differ in their homodimeric association and active-site geometry. The vaccinia TMP kinase dimer arrangement is orthogonal and not antiparallel as in human enzyme. This different monomer orientation is related to the presence of a canal connecting the edge of the dimer interface to the TMP base binding pocket. Consequently, the pox enzyme accommodates nucleotides with bulkier bases, like brivudin monophosphate and dGMP; these are efficiently phosphorylated and stabilize the enzyme. The brivudin monophosphate-bound structure explains the structural basis for this specificity, opening the way to the rational development of specific antipox agents that may also be suitable for poxvirus TMP kinase gene-based chemotherapy of cancer.

Pharmacodynamics of cidofovir for vaccinia virus infection in an in vitro hollow-fiber infection model system

Variola major virus remains a potent weapon of bioterror. There is currently an investigational-new-drug application for cidofovir for the therapy of variola major virus infections. Stittelaar and colleagues compared the levels of effectiveness of postexposure smallpox vaccination (Elstree-RIVM) and antiviral treatment with cidofovir or an acyclic nucleoside phosphonate analogue 6-[2-(phosphonomethoxy)alkoxy]-2,4-diaminopyrimidine (HPMPO-DAPy) after lethal intratracheal infection of cynomolgus monkeys with monkeypox virus, a variola virus surrogate. Their results demonstrated that either compound was more effective than vaccination with the Ellstree vaccine (K. J. Stittelaar et al., Nature 439:745-748, 2006). An unanswered question is how to translate this information into therapy for poxvirus infections in people. In a proof-of-principle study, we used a novel in vitro hollow-fiber infection model system to determine the pharmacodynamics of vaccinia virus infection of HeLa-S3 cells treated with cidofovir. Our results demonstrate that the currently licensed dose of cidofovir of 5 mg/kg of body weight weekly with probenecid (which ameliorates nephrotoxicity) is unlikely to provide protection for patients intentionally exposed to Variola major virus. We further demonstrate that the antiviral effect is independent of the schedule of drug administration. Exposures (area under the concentration-time curve) to cidofovir that will have a robust protective effect will require doses that are 5 to 10 times that currently administered to humans. Such doses may cause nephrotoxicity, and therefore, approaches that include probenecid administration as well as schedules of administration that will help ameliorate the uptake of cidofovir into renal tubular epithelial cells need to be considered when addressing such treatment for people.

Mechanism of antiviral drug resistance of vaccinia virus: identification of residues in the viral DNA polymerase conferring differential resistance to antipoxvirus drugs

The acyclic nucleoside phosphonate (ANP) family of drugs shows promise as therapeutics for treating poxvirus infections. However, it has been questioned whether the utility of these compounds could be compromised through the intentional genetic modification of viral sequences by bioterrorists or the selection of drug resistance viruses during the course of antiviral therapy. To address these concerns, vaccinia virus (strain Lederle) was passaged 40 times in medium containing an escalating dose of (S)-1-[3-hydroxy-2-(phosphonomethoxypropyl)-2,6-diaminopurine [(S)-HPMPDAP], which selected for mutant viruses exhibiting a approximately 15-fold-increased resistance to the drug. (S)-HPMPDAP-resistant viruses were generated because this compound was shown to be one of the most highly selective and effective ANPs for the treatment of poxvirus infections. DNA sequence analysis revealed that these viruses encoded mutations in the E9L (DNA polymerase) gene, and marker rescue studies showed that the phenotype was produced by a combination of two (A684V and S851Y) substitution mutations. The effects of these mutations on drug resistance were tested against various ANPs, both separately and collectively, and compared with E9L A314T and A684V mutations previously isolated using selection for resistance to cidofovir, i.e., (S)-1-[3-hydroxy-2-(phosphonomethoxypropyl)cytosine]. These studies demonstrated a complex pattern of resistance, although as a general rule, the double-mutant viruses exhibited greater resistance to the deoxyadenosine than to deoxycytidine nucleotide analogs. The S851Y mutant virus exhibited a low level of resistance to dCMP analogues but high-level resistance to dAMP analogues and to 6-[3-hydroxy-2-(phosphonomethoxy)propoxy]-2,4-diaminopyrimidine, which is considered to mimic the purine ring system. Notably, (S)-9-[3-hydroxy-2-(phosphonomethoxy)propyl]-3-deazaadenine retained marked activity against most of these mutant viruses. In vitro studies showed that the A684V mutation partially suppressed a virus growth defect and mutator phenotype created by the S851Y mutation, but all of the mutant viruses still exhibited a variable degree of reduced virulence in a mouse intranasal challenge model. Infections caused by these drug-resistant viruses in mice were still treatable with higher concentrations of the ANPs. These studies have identified a novel mechanism for the development of mutator DNA polymerases and provide further evidence that antipoxviral therapeutic strategies would not readily be undermined by selection for resistance to ANP drugs.

Antiviral activities and phosphorylation of 5-halo-2'-deoxyuridines and N-methanocarbathymidine in cells infected with vaccinia virus

BACKGROUND

The antipoxviral activities and phosphorylation of N-methanocarbathymidine ([N]-MCT) and four 5-halo-2'-deoxyuridines, namely 5-fluoro-(FdU), 5-chloro-(CldU), 5-bromo-(BrdU), and 5-iodo-(IdU) derivatives, were explored.

METHODS

Antiviral activities and nucleoside metabolism were determined in C127I mouse, LLC-MK2 monkey, and A549 human cells infected with thymidine-kinase-containing and -deficient (TK+ and TK-) vaccinia (WR strain) viruses.

RESULTS

The antiviral potencies of CldU, BrdU and IdU were increased 16-26-fold in LLC-MK2 cells infected with TK+ compared with TK- virus infections, but enhancement of activity was much less in the other cell lines. (N)-MCT was nearly equally active against TK+ and TK- viruses in the three cell lines. Antiviral activity of FdU was associated with cytotoxicity. Uninfected and infected cells metabolized compounds to mono-, di- and triphosphates. The thymidine, BrdU and IdU triphosphate levels were higher in C127I and LLC-MK2 cells infected with TK+ than with TK- virus. (N)-MCT monophosphate levels were much higher in TK+ virus-infected cells, but without corresponding increases in (N)-MCT triphosphate. Furthermore, TK+ virus infections did not appreciably alter (N)-MCT triphosphate levels in other mouse (L929), monkey (MA-104 and Vero) and human cell lines (A549). Antiviral potency of the compounds was greater in C127I than in LLC-MK2 cells, yet lower intracellular triphosphate levels were found in C127I cells.

CONCLUSION

We conclude that viral TK plays an important role in increasing the antiviral potencies of these compounds in some cell lines, but minimally in others. These findings may have implications in treating infected animals with compounds that are dependent upon poxvirus TK for their activation, because viral TK activity may vary greatly due to cell type.

Inosine monophosphate dehydrogenase (IMPDH) as a target in drug discovery

Inosine monophosphate dehydrogenase (IMPDH) is a key enzyme of de novo purine nucleotide biosynthesis and is viewed as an important target in the quest for discovery of drugs in the antiviral, antibacterial and anticancer therapeutic areas. This review focuses on the medicinal chemistry, drug discovery and chemical biology of IMPDH. Examples of IMP and cofactor site-directed inhibitors, allosteric inhibitors and isoform-selective inhibitors are presented. Comparison of IMPDHs from different organisms is also made to facilitate the design of species-selective IMPDH inhibitors for drug discovery. Special emphasis in the review is placed on IMPDH from Mycobacterium tuberculosis.